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How well do animals see? Well that’s a good question. Visual function involves a combination of many factors, including: the field of view, depth perception (ability to judge distances), acuity (focusing ability), perception of motion, and color differentiation. All of these functions must then be integrated by the brain to produce useful vision.
The eyes of dogs and cats have many of the modifications typically seen in animals which evolved as nighttime hunters. The pupil functions much as the aperture for a camera and can dilate for maximal light capturing ability in dogs and cats. In addition, there is a reflective layer under the retina called the tapetum which serves to intensify vision in dim light. The “mirror” effect of the tapetum results in the “eye shine” observed when an animal looks into a car’s headlights.
And dogs do see in color, sort of, but their perception of color is not the same as it is for people. They cannot distinguish between red, orange, yellow or green. They can see various shades of blue and can differentiate between closely related shades of gray that are not distinguishable to people.
In a study published in the Journal of the American Veterinary Medical Association, researchers dissected canine retinas and found many color-sensitive cones that indicated that there was at least an anatomical potential for color vision in dogs. The researchers also used behavioral discrimination testing and electrical photo tests to determine the light wavelengths that stimulated these cones. The dogs in the study could not differentiate between middle-to-long wavelengths of light, which to people appear as green, yellow, orange or red.
When comparing dog and human vision, people are better at depth perception, color perception and seeing minute details of an object. Dogs are better at seeing in dim light, responding to an image rapidly and detecting the slightest motion. They also have better peripheral vision.
Dogs have eyes which are placed on the sides of the head, resulting in a visual field of 240 degrees compared with the human field of 200 degrees. The central, binocular field of vision in dogs and cats is approximately half that possessed by humans.
And as many a dog owner knows one of the most common diseases which can affect the clarity of the eye is formation of a cataract within the lens. Cataracts in dogs are most often inherited and may affect dogs at any age. As the lens becomes progressively opaque, an animal’s vision deteriorates so that only light and dark perception exists. Surprisingly, animals function relatively well in familiar surroundings, even with severe vision impairment. This illustrates the ability of dogs and cats to depend heavily on their other senses, namely smell and hearing.
Of Insects, Birds…
For centuries, humans were in the dark about what and how other animals see. Recent scientific investigation has revealed an amazing world of vision diversity. Like the dragonfly, its brain works so fast that it sees movements as if in slow motion. Or the pigeon, which is capable of detecting more subtle gradations of color than the most advanced computer program.
Horses have an amazing range of vision, that is, except for what is right in front of them. They literally can’t spot whatever is between their eyes and therefore directly ahead due to their binocular vision. This is why they so often look down as they walk.
Many birds can see differently. Pigeons, for example, can see literally millions of different hues and are thought to be among the best at color detection ability of any animal on earth. They have many more cones than humans in their eyes, thus accounting for the ability to see at least five spectral bands.
Cats and dogs do not have strong vision. They rely on scent and sound primarily as their sensory detection. Cats in particular have weak vision. They are color blind, more so than dogs.
Snakes have two sets of eyes. One set is the normal eyes that you see, and they detect color quite well. But they also have vision pits that detect heat and “see” living creatures like an infrared detector. There is no getting away from a snake once you’re spotted.
October 1, 2016